Translation:If I ate more, I would be stronger.
I am not completely sure what subjunctive means, I am only recalling Duos introduction saying that לו "are used only for events that haven't or can't happen". And in english conditionals the if-part doesnt come with "would", so you need to omit it as in "If I had eaten more,..."
Lu is for things that can't happen OR for things that are wished for, hoped for, or theoretical.
Indicative: I was rich.
Subjunctive: If I were rich.
(notice how the verb changes form in English? The first one is said as a fact. The second one is a hope or wish.)
No, lū is already attested in Akkadian as a precative particle may... and also preserved in Arabic لَوْ [law] if (contrary to fact), which are the two main meanings לוּ has in Classical Hebrew. Rabbinical Hebrew combined אִלוּ (אִם + לוּ) with לֹא in its tendency to use bigger words for judical discussions, where negative contrary to fact sentences became more usual.